I have downloaded DLC but it won't install

I can't locate where my saves on the base game are located. I loaded the packs through Steam so it seems likely that the base game that I'm playing is not there, but I've been playing through Steam so I definitely have the base game on there. 

I think the answer is both, but they appear to be working in sync when I'm trying to play the base game and with another game pack that I installed via the same methods a couple of months ago, so I'm not sure why there are issues now.

I've read through that thread already in my hunt to fix it, but I don't have the CAS demo installed and when I searched for the packs they appeared as downloaded where my other one (which does work in game) is located - in the Steam folder. 

The error message essentially says that the packs can't locate the base game and therefore stopped installing, but the base game is there as best I can tell. Is there a way of identifying for sure? Steam thinks I've got the base game when I try and play and I can't get it again because I already own it according to the Steam app. 

I've uninstalled Sims 4 from Steam and redownloaded but the error didn't rectify itself.